The present invention relates to a device for heating long, thin items in a continuous pass by means of electromagnetic induction. In particular, it relates to the inductive heating of thin strip metallic items, such as sheets and strips.
The heating of metallic items by means of electromagnetic induction is widely used, on an industrial basis, for some types of items. However, the inductive heating technique is limited to items of modest size, normally not exceeding the size of steel billets. Furthermore, it is reserved to the discontinuous processing of separate items on an individual basis.
In contrast to this technique, long thin items, such as metal sheets, are usually heated in fuel or gas furnaces in a continuous process. The heating time constant of the metal sheets in the furnace often imposes a requirement of rather long duration for the passage of each sheet through the furnace. This results in the need for a substantial amount of space in terms of length inside the furnace, and hence considerably increases the size of the furnace.
One of the requirements for continuous processing of sheet materials in furnaces is that the material be fed through the furnace at a constant rate. In order to accomodate for this requirement, it becomes necessary to provide a substantial amount of space for storage for strips or sheets of material to be heated. Consequently, the size of the processing installation, and hence its cost, are further increased. In addition, the energy efficiency of furnaces used in this process is sometimes rather poor.
The most common devices for induction heating which are presently used are provided with fixed induction coils for generating variable magnetic fields. These coils use a large amount of reactive energy and thus, in view of the large amounts of power consumed, require compensating devices such as condenser sets. The current requirements of these devices also lead to important additional losses through Joule's effect. These losses appreciably harm the yield of the installations and thus further diminish their efficiency.
In contrast to prior art induction heating techniques, the present invention uses magnetic fields of constant strength which are mechanically moved. This feature of the invention considerably reduces the consumption of reactive energy and considerably improves efficiency.
Induction heating devices, using constant strength magnetic fields, have previously been proposed. However, they are neither conceived nor adapted for heating thin items in a continuous pass.